Page 22 - MetalForming May 2014

Advanced High Strength Steels
Fig. 2—The hot-forming sequence creates complex stampings with very high strength and no springback issues.
that proves difficult to measure exper- imentally.
Extensive research has been con- ducted to develop some means to pre- dict edge-stretch limits. The traditional FLC was never intended to be used for stretch limits of cut, sheared or punched edges. During cutting, the steel severely deforms and the n-value decreases to an unknown value. This deformation can extend back from the cut edge by one-half of sheet thickness. DP and TRIP steels contain islands of hard martensite that act as crack ini- tiators to lower the FLC even more. Improving the cutting parameters– clearance, and cutting-blade angle and orientation–provides some improve- ment. Other research approaches attempt to alter the microstructure of the steel to increase stretchability.
Another forming mode affected by AHSS is stretch-bend limits with tight bend angles. With the outer surface of the bend in tension, it exhibits reduced stretchability because the hard martensite
This new unit to the guidelines cov- ers the many forming properties obtained from tensile tests. It defines each property and explains how it changes as the tensile test progresses. Properties not acquired from a tensile test also are discussed.
The most important property for forming is work (strain) hardening, defined by the n-value. After stabilizing the beginning of the tensile test, con- ventional low- and high-strength steels exhibit constant n-values. In contrast, dual-phase and TRIP steels have n-val- ues that change during deformation. This requires obtaining the instanta- neous n-values over the course of the test—these values better-reflect mate-
rial stretchability.
Deformation Limits: Forming limit
curves (FLC) have been used for decades to predict the maximum amount of stretch available for different combinations of major and minor sur- face strains. While the details of these curves vary by global location, the end purpose remains the same—do not allow stampings to be made without a sufficient safety margin. The same FLC procedure used for conventional steels can be used for DP steels. However, the properties of TRIP steels change during forming, which cancels most FLC calculations. With a total elonga- tion around 5 percent, very high- strength martensite has a very low FLC
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